Ask 100 people who are desperate to enter the United States about their pain points, how they currently plan to enter the country and how they would use a wall.

Use these insights to build a Minimum Viable Wall (MVW) designed to prevent desperate people from getting into the U.S. (Note: Make sure your MVW is long enough so that people cannot just walk around it.)

Invite 100 desperate people to try and bypass the wall.

Watch how the MVW performs and pay particular attention to how people are able to breach it, i.e. scaling, penetrating and tunneling. (It may take some time, but they will figure it out, you can count on it. Remember they are desperate.)

Repeat steps 3 and 4 above at least 10 times, each time improving the Wall based on your observations and insights.

Eventually you will have a prototype Wall that meets your Minimum Viable Objective, i.e. it prevents 25% of desperate people from entering the country. Develop a plan to handle the 75% who do enter, i.e. detain, deport or help place them in menial jobs.

Build and install the rest of the Wall.

Repeat step 4.

Update the Wall, adding features to prevent even more desperate people from entering the U.S.

Repeat steps 8 and 9 until the Wall prevents 100% of desperate people from entering the country. Make sure to tell companies that they will no longer have access to cheap labor to do menial jobs.

Sell advertising on both sides of the Wall to pay for construction and maintenance. Give free ads to compensate companies for the loss of cheap labor.